1. Field of the Invention
The present invention relates generally to well logging apparatus and methods, and pertains in particular to improved telemetering apparatus and methods for receiving and transmitting information from well logging tools to the earth's surface.
2. The Prior Art
In most well logging applications, full advantage of the information-gathering capabilities of the downhole equipment can be realized only through processing of the downhole data at the earth's surface, whether this be done at the well site or remotely. It is important, therefore, that the downhole data be transmitted to the surface as free as possible of noise or other disturbing effects and, if possible, without loss of information-bearing content. Available logging cables, however, have limited frequency responses beyond which further information-bearing signals cannot be transmitted in an effective way. The result is that in certain logging situations the downhole data is lost or distorted in transmission, with consequent degradation in the quality of the information provided by the surface processing equipment. This problem is particularly acute in logging tools in which the downhole data is generated at high instantaneous rates, such as gamma ray spectroscopy tools for instance, or where plural types of data are generated concurrently and compete for cable time.
Various systems for telemetering well logging data have been proposed in the past to overcome these difficulties in data transmission. One such system is described in U.S. Pat. No. 3,559,163, granted on Jan. 26, 1971 to R. J. Schwartz, and assigned to the assignee of this application. The Schwartz system includes a provision for downhole storage of output pulses from one or more information-gathering transducers and for interrogating the stored data at a rate enabling transmission of the data from storage to the earth's surface within the frequency response capacity of the cable. Where the logging tool includes a plurality of transducers, provision is also made for storing the data from each transducer and for applying it to the cable on a time-sharing basis. Although the Schwartz system represents a useful prior art approach in the area of well tool information transmission, other telemetering methods and apparatus having still further improved characteristics and advantages are desired.